there are different difficulties. intermediate is quite easy, advanced is challenging but possible, extreme is very hard (of course, still possible) and requires lots of time if you're playing with randoms. if your team knows what to do, even expert can be easy, but to learn that, i'd suggest trying advanced mode first (or intermediate if you prefer rust/blood botkillers over silver/gold ones)

yeah - the top advice is given already - try Oil Spill first as its the simplest mission and work up the skill levels from there. The end reward is worth it too as the rusty BK weapons look pretty sweet